import numpy as np
import cv2 as cv
img =np.ones((512,512,3),np.uint8)*255

# Draw a diagonal blue line with thickness of 5 px
startPt =(0,0)
endPt=(img.shape[1]-1,img.shape[0]-1)
color=(255,0,0)
thinknessPx=5
cv.line(img,startPt,endPt, color,thinknessPx)
cv.imshow("img",img)
cv.waitKey(0)
cv.destrovAllWindows()